1. Field of the Invention
The invention relates to the manufacture of anodes which can be operated in a manner neutral to the surroundings, especially of anodes intended for cooperation with an electrolyte of a cell for production of aluminum by fusion electrolysis. The anodes have a layer applied on the anode surface consisting of a material which protects the anode surface against surrounding influences and dissolves in the electrolyte.
The cathodically connected pot of a fusion electrolysis cell for production of aluminum contains molten aluminum, and an electrolyte which floats on the aluminum and contains aluminum oxide; the electrolyte on its side directed towards the atmosphere forms a solid crust, and this in turn is covered with a layer of alumina (Al.sub.2 O.sub.3) for periodical enrichment of the electrolyte and for thermal insulation of the bath. Anodes consisting of artificial carbon penetrate the alumina layer and the crust and extend into the electrolyte. The crust usually does not sealingly surround the circumference of each anode, but a gap forms around the anode circumference because of rising gases and other influences. The consumption of an anode during the operation of the cell, also known as burning away, which is made up of a primary and secondary burning, is based upon two oxidation mechanisms which should be considered separately for the present purpose.
In the primary burning, the oxygen released from the aluminum oxide during the fusion electrolysis attacks the carbon of the anode with formation of a gas mixture of carbon dioxide and monoxide, which mostly rises along the anode surfaces and through the gap.
This reaction, which brings about the majority of the burning away, occurs exothermically with heating of the electrolyte and reduction of the energy necessary for the electrolysis. This primary burning is unavoidable with carbon anodes.
On the contrary, this is not the situation with the secondary burning, which is based on another oxidation mechanism and which impairs economic cell operation. The present invention is concerned with resisting this secondary burning. The cause of secondary burning is as follows.
The temperature of the bath in the cell (i.e. the cathode pot) lies at about 950.degree. to 980.degree.C, and this source of heat imparts a heat content to the carbon anodes, so that a gradient is established between the side of the anode facing towards the bath and facing away from the bath. Corresponding to this heat content of the anodes, the surfaces of the anodes exhibit a corresponding temperature gradient between a maximum of 980.degree., and 400.degree.C. At the same time the part of the anode extending out of the bath is surrounded with an atmosphere consisting of air, together with a gas mixture of carbon monoxide and dioxide and smaller quantities of vaporised fluorides rising mainly through the gap around the anode and burning in the air. Encouraged by the high temperature of the anode, this atmosphere has an oxidising effect, thus encouraging burning away. The burning reactions which thus occur produce, in contrast to the oxidation mechanism causing the primary burning, no contribution to the bath heating and thus to reduction of energy, but they produce unproductive losses of carbon which can, for a definite quantity of aluminum produced, amount to up to 8 % of the total consumption of anode carbon.
2. Description of the Prior Art
It is known to manufacture an anode which during its operation is substantially neutral to its surroundings, i.e. free of secondary burning away. This involves the application of a coating on the anode surfaces hindering oxidising media from attack on the anode surfaces extending out of the cell bath. In this method, an anode cladding is produced by casting of aluminum on the anode surfaces. In order that it can attain its intended function, this anode cladding has thickness of at leat one cm, but preferably several cms, and this requires considerable expenditure. A layer of this kind cannot be applied on the anode surface without moulds or similar means, which require a series of working operations which introduce expense and are mostly manual. To avoid the introduction of oxidation products, special measures must be observed as the melt is conducted to the casting process. Besides, one must ensure that the wetting of the carbon by aluminum, which is anyway not good, is not still further impaired by inappropriate procedure. As mentioned initially, the secondary burning amounts to up to 8% of the total consumption of carbon for a given quantity of aluminum produced, and if an economic benefit is to be obtained from elimination of the secondary burning, then the costs devoted to this elimination must be held within the range of the above-mentioned fraction of the carbon costs. The known method is economically unattractive.